This invention relates to semiconductor devices which are encapsulated in a filled organic resin.
Semiconductor devices are commonly encapsulated to protect them from exposure to environmental hazards such as air, moisture, chemicals, dust and light, and to provide them with greater physical strength. The encapsulating material must be an electrical insulator, and for that reason ceramic materials and, more typically, formulated organic resins are used for that purpose. The most common of these resins are thermosetting epoxy resins.
Inert fillers are almost always present in resin systems used to encapsulate semiconductor devices. The most common filler is fused silica. Fillers such as fused silica decrease the coefficient of thermal expansion (CTE) of the resin system to more closely match that of the encapsulated metal lead frames and semiconductor dies. This prevents damage to the device from thermal cycling and reduces compressive and shear forces on the surface of the semiconductor.
Semiconductor devices generate heat as they operate. As these devices become more complex with more closely spaced elements on their surface, removal of this heat becomes a significant problem. A major disadvantage of silica-filled epoxy encapsulants is their low thermal conductivity, which inhibits rapid dissipation of heat from the embedded semiconductor. For this reason, methods are desired by which improved heat dissipation can be achieved.
One approach has been to incorporate metal heat sinks or metal heat spreaders into the encapsulated devices. These provide a means for rapid heat dissipation, but interject additional parts and manipulative steps into the manufacturing process, which substantially increases the cost.
Another approach has been to encapsulate the semiconductor in a resin loaded with a highly thermally conductive filler. For example, aluminum oxide (alumina), silicon carbide, silicon nitride and aluminum nitride fillers have all been tried. All of these improve the thermal conductivity of the encapsulant relative to fused silica. Unfortunately, all suffer from significant disadvantages. Some substantially increase the CTE of the encapsulant, making the device more susceptible to damage from thermal cycling and die stress. Others, like silicon carbide and alumina, are very hard materials which can cause excessive erosion of the molding equipment used to encapsulate the devices. Silicon carbide has a high dielectric constant, which makes it unsuitable for most applications. Silicon nitride and alumina impart better thermal conductivity to the encapsulant than does fused silica, but not as much as desired, and silicon nitride is hydrolytically unstable as well. Aluminum nitride is hydrolytically unstable, and can react with ambient moisture to cause dimensional instability, and generate alkaline by-products which attack the metal conductor on the semiconductor surface.
Accordingly, it would be desirable to provide an encapsulated semiconductor device with greater ability to dissipate heat than devices encapsulated with a fused silica-filled resin. It would be further desirable if such a device were encapsulated in a material having a CTE close to that of silicon, and which does not require the incorporation of metal heat sinks and/or metal heat spreaders into the device.